ddfs_util: ignore unreadable disks for gc.
ddfs_util: ignore unreadable disks for gc.

If a volume cannot be read, we still can proceed with the gc but we should
assume all nothing is in that volume.

oldmantaiter/disco,seabirdzh/disco,oldmantaiter/disco,pooya/disco,simudream/disco,discoproject/disco,ktkt2009/disco,discoproject/disco,ktkt2009/disco,seabirdzh/disco,seabirdzh/disco,ktkt2009/disco,pooya/disco,simudream/disco,mozilla/disco,pombredanne/disco,beni55/disco,beni55/disco,beni55/disco,seabirdzh/disco,mozilla/disco,simudream/disco,mwilliams3/disco,ErikDubbelboer/disco,mozilla/disco,oldmantaiter/disco,beni55/disco,mozilla/disco,ktkt2009/disco,discoproject/disco,ErikDubbelboer/disco,pooya/disco,pombredanne/disco,ErikDubbelboer/disco,mwilliams3/disco,simudream/disco,pombredanne/disco,seabirdzh/disco,ErikDubbelboer/disco,beni55/disco,oldmantaiter/disco,ErikDubbelboer/disco,mwilliams3/disco,simudream/disco,discoproject/disco,oldmantaiter/disco,pombredanne/disco,mwilliams3/disco,pooya/disco,pombredanne/disco,ktkt2009/disco,discoproject/disco,mwilliams3/disco